This invention is concerned with film cartridges, and more particularly with endless-roll film cartridges employed in motion picture projectors.
U.S. Pat. No. 3,244,471, granted Apr. 5, 1966, assigned to the same assignee as the present invention, and incorporated herein in its entirety by reference, discloses an endless-roll film cartridge for use with motion picture projectors, for example. The cartridge comprises a housing containing the film roll, which includes a coil of film in a compartment of the housing and a loop extending between the outer and inner convolutions of the coil. A segment of the loop is trained past an aperture in the housing at which film perforations are engaged by a film drive claw, the film being held against the aperture by a pressure plate. The coil of film is moved solely by compressional forces in the film segment returning to the coil and by tensile forces in the film segment leaving the coil, such forces being produced in the loop by the action of the drive claw. When the film is driven at very low speeds (of the order of two to three frames per second or slower, for example), there is often a cyclical variation in stepping distance (tick-tocking) as the claw advances the film from frame to frame. It has been found that the problem occurs as the main body of the film in the cartridge completes its advance from one step and presents an inertia load to the next advance. It has been discovered that the condition of this inertia load (tightness and placement of the coil and the amount of slack in the loop) is affected by the condition of the inertia load prior to the film advance. Thus it is possible, in some instances of cartridge loading conditions (film loop total length, film stiffness, lubrication, pressure plate force, etc.) that cannot be precisely controlled, for the system to shift the inertia loading condition in a rhythmic way. The reaction on the claw causes the film to be advanced a slightly shortened step and then a slightly lengthened step, with each step variation the result of the previous variation, so that the projected image moves up and down alternately as much as 10 to 15 percent of its height on successive steps.